1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer for forming and outputting an image on a paper sheet by utilizing the electrophotography.
The present invention relates further to a paper bundling apparatus and a paper bundling method for making a bundle of paper sheets having images produced.
2. Description of the Prior Art
In the image forming apparatus utilizing the electrophotography, generally speaking, the image is produced on the paper sheet by forming an electrostatic latent image corresponding to image signals on a photosensitive member, by transferring a toner image developed from the latent image to the paper sheet and then by fixing the transferred toner image on the paper sheet.
Here in this image forming apparatus, usually, paper sheets stacked on a feed tray are fed one by one and are transferred to a toner image transfer position. During the transfer, however, the paper sheets being transferred may be inclined by various factors (such as an assembling accuracy of mechanical parts, or a slipping phenomenon). If, in this case, the paper sheets are delivered at the inclination to the transfer position, the image is produced with a shear on the paper sheets.
In some image forming apparatus, therefore, there is assembled in a paper transfer line a mechanism for correcting the shear due to the inclination (as will be called the "skew") of the paper sheet being transferred. Specifically, as shown in FIG. 14, the mechanism is provided with two paper transfer rollers A and B which are disposed along the transfer direction of paper sheet P. The position of this paper sheet P is corrected with respect to the leading end of the paper sheet P being transferred, by bringing the paper sheet P conveyed by the upstream paper transfer roller B into abutment against the downstream paper transfer roller A stopped, so that the skew component may be corrected.
In the image forming apparatus described above, however, the paper sheet P takes a generally straight advancing direction if its leading end is correctly cut at about 90 degrees with respect to the transfer direction, as shown in FIG. 15(a). If the leading end of the paper sheet P is not correctly out at about 90 degrees, as shown in FIG. 15(b), the paper sheet P is angularly changed by correcting the skew component so that it is conveyed at the inclination. In short, the aforementioned image forming apparatus may fail to correct the skew regularly depending upon the cut angle of the leading end of the paper sheet.
This failure may invite the following problems when an image is to be produced on a paper sheet.
The paper sheet to have an image produced by the image forming apparatus frequently changes its shape delicately from a rectangular shape into a parallelogram due to the cutting error or the like. As shown in FIG. 16(a) or 16(b), therefore, the paper sheet P is gradually shifted sideway by correcting the skew when it is transferred. Here, the position to produce an image G is always generally fixed irrespective of the shift of the paper sheet P so that the image G goes out of position on the paper sheet P.
When the cutting accuracy of the paper sheet P is poor, more specifically, it is conceivable that a shear occurs in the paper sheet P and in the position to produce the image G so that the image G produced, as shown, extrudes from the paper region thereby to degrade the quality of the image production. Even if the paper cutting accuracy is not so poor, on the other hand, the region set near the paper ends to bear no image (as will be called the "marginal regions") may increase/decrease depending on the position of the paper end. If, in this case, the marginal regions become smaller a certain limit, the toner material may scatter to stick to the portions of the paper other than the image forming portions thereby to degrade the image forming quality seriously.
If the correction of the skew component is adversely affected by the cutting angle at the leading end of the paper sheet, on the other hand, the following problems may occur in the image forming apparatus having a function to produce images on the two sides of a paper sheet.
In this image forming apparatus, images are usually produced on the two sides of a paper sheet by forming an image at first on one side and then on the other side after the paper sheet was reversed. At this time, however, most image forming apparatuss are restricted to reverse the front and back sides of the paper sheet while interchanging the leading and trailing ends at the transfer time of the paper sheet by the size and cost of the apparatus and so on.
When the paper sheet P having a shape, as shown in FIG. 14(a), is to be transferred, therefore, it is transferred generally straight, as shown in FIG. 14(a), for one side but with a skew, as shown in FIG. 14(b), for the other side. As a result, the positional relation between the produced images and the paper sheet is shown in FIG. 17(a) for one side but in FIG. 17(b) for the other side. When the paper sheet P bearing the images on its two sides is peered into, therefore, it is found, as shown in FIG. 17(c), that the image G1 on the surface and the image G2 on the back shear depending upon the cutting accuracy of the paper sheet P.
When printed matters are utilized as a pamphlet or when a double-side print such as a calling card is to be made by forming the same image on a plurality of portions of the same side of a paper sheet and then by cutting the paper sheet, for example, the shear between the front and back sides invites the shear from the center, extrusion or distortion of the image on the other side, if the paper sheet is cut with reference to one side. When the paper sheets having the produced images are bundled into a book, for example, on the other hand, the positional relation between the paper sheets and the images is dispersed to provide a poor appearance so that the printed matters lose the commercial value as the book.